Modern wind turbines are usually designed with variable rotational speed and have a converter. The generator can therefore turn with a rotational speed determined by the wind strength and in the process generate electrical energy with correspondingly speed-dependent frequency. This variable-frequency electrical energy is converted by the converter so that it is suitable for feeding into a fixed-frequency supply network (usually 50 Hz). Converters of this kind usually have two inverters which are connected by a link circuit. In doing so, one of the inverters is connected to the network and, in operation, is supplied with the network frequency (network-side inverter), while the other inverter (machine-side inverter) is connected to the generator, wherein the frequency applied thereto is determined among other things by the rotational speed of the wind rotor. Converters of this kind can be designed as full converters or partial converters, the latter in particular in combination with a doubly fed asynchronous machine. However, with the increasing spread of wind turbines and the rise in installed reactive power, the demand is no longer for a simple feeding of active power into the electrical network alone; instead a mode of operation of the wind turbines which performs additional services for the network is desired. The so-called system services for the network in particular include the feeding of reactive power, for example for supporting the network at reduced voltage or network frequency. In so doing, particularly in the case of the doubly fed asynchronous generators which are often used with wind turbines with higher power, the problem can occur that the converter, which is usually rated for only one third of the total electrical power of the wind turbine, is no longer able to achieve the additional currents required to support the network. This applies particularly when unfavorable operating conditions, which in any case lead to increased currents, prevail due to undervoltage or low network frequency. Furthermore, additional complications can arise due to additional requirements such as, for example, the requirement for low-noise operation and, frequently associated therewith, operation in the region of synchronous speed.
For better utilization of the converter, it is known to determine the reactive power current for the network-side inverter linearly as a function of the total reactive power demand. Although this improves the utilization of the converter with regard to reactive power recovery, there is an associated risk of overload, above all in the critical operating situations close to the synchronous operating point. Furthermore, it has been proposed to dynamically vary the distribution of reactive power between the two inverters of the converter during operation. This must be carried out as a function of whether certain pre-defined conditions are fulfilled in order to relieve the reactive power on the inverter with the higher active power loading. It has been shown that the proposed concept gives rise to certain difficulties in balancing, and frequently only a mode of operation at the nominal point at rated speed, rated power and rated network frequency can be balanced.